Electrochromism means the phenomenon by which color changes according to a potential difference of an applied electric field. The most representative electrochromic materials are such inorganic metal oxides as WO3, Ir(OH)x, MoO3, V2O5, TiO2, NiO and LiNiO2, and such organic compounds as viologen, anthraquinone, phenothiazine, polyaniline, polypyrrole and polythiophene. The electrochromism was first found in 1961, however no electrochromic devices have been commercially mass-produced so far because of disadvantages of difficulty in multiple color expression, low speed of coloring and decoloring, an after-image remaining after decoloring, and decomposition of an organic compound by the repeated coloring-decoloring.
To overcome the problems of using an organic compound for the device, an inorganic metal oxide has been used as an electrochromic material as an alternative. Owing to the inorganic metal oxide, life-time, UV stability and coloring-decoloring speed have been significantly improved. WO3, one of the representative inorganic metal oxides, is colorless in oxidation state and is colored in reduction state, so that studies have been going on about WO3 as a promising electrochromic material for a reductive electrode.
The inorganic electrochromic material for an oxidative electrode is exemplified by Ce—TiO2, ATO (Sb-doped SnO2), NiO, LixNi1−yO, etc. However, with these inorganic electrochromic materials, the sol-gel reaction temperature has to be 400° C. or higher, actuation voltage in the device is high, coloring-decoloring speed is very slow, and optical properties are not very good. In the meantime, lithium nickel oxide (LixNi1−yO) has high transmittance, excellent electrochromic property, low driving voltage and fast reaction speed, so that recent studies have been focused on it as an electrochromic material for an oxidative electrode. The structural analysis of the lithium nickel oxide (LixNi1−yO) synthesized by sol-gel reaction has already been made and thus the mechanism of coming in and out of Li+ and the structural change thereby have been reported.
The lithium nickel oxide (LixNi1−yO) is generally prepared by sol-gel reaction, sputtering or pulse laser deposition, etc. Even though, lithium nickel oxide produced by one of these methods is good enough for the use as an electrochromic material, it still has a few problems for being used as an electrochromic material formed on the conductive substrate.
First, Ni2+ and Ni3+ are co-existed in the structure of the lithium nickel oxide (LixNi1−yO) electrode prepared by sputtering, making it unstable. So, if this lithium nickel oxide is used as a electrochromic material for an electrochromic device, the electrochromic range will be very narrow since the lithium nickel oxide will not be completely oxidized or reduced. In particular, incomplete reduction makes matter worse with making the product still colored and unclear. Therefore, after assembling an oxidative electrode containing the lithium nickel oxide (LixNi1−yO) layer with the reductive electrode to prepare electrochromic device, it is required to apply oxidative-reductive voltage repeatedly on the electrochromic device prepared thereby in order to decolor the lithium nickel oxide (LixNi1−yO) completely. But, if the lithium nickel oxide (LixNi1−yO) layer is approximately 150 nm or more in thickness, hundreds of oxidative-reductive voltage applications are required to decolor the lithium nickel oxide (LixNi1−yO) completely. Even after hundreds of decolorization processes, complete decolorization is not guaranteed.
Besides, the lithium nickel oxide (LixNi1−yO) deposited by sputtering method exhibits poor adhesion onto the surface of FTO (Fluorine-doped Tin Oxide) glass. Owing to repeated oxidation-reduction after completing the electrochromic device, the deposited lithium nickel oxide is changed into NiOx or NiOOH, which is then separated from the electrode surface or generates air bubbles in electrolytes.